Field of the Invention
This invention relates to a semiconductor light exposure device applied in a stepper used in microprocessing of a semiconductor device, a projection exposure device used in this semiconductor light exposure device and a method for producing a circuit pattern in which the projection light exposure device is applied.
Up to now, researches have been conducted towards a shorter wavelength of a light source of a light exposure device used in microlithographic processing of a semiconductor device.
It should be noted that the fifth harmonics, that is a laser beam of a wavelength of 213 nm of the neodymium:yttrium aluminum garnet (Nd:YAG) laser generating a laser beam of the wavelength of 213 nm, has a high energy peak strength and is small-sized and inexpensive, so that it is highly promising as a laser light source of the next-generation light exposure device. In addition, it is of a narrow spectral width and hence is less susceptible to chromatic aberration.
However, the ultraviolet laser beam, obtained on wavelength conversion of the laser beam generated on oscillation by the Nd:YAG laser, is high in monochromaticity and hence narrow in spectral width, so that it exhibits high coherency. It is noted that coherency is an index specifying over which distance and time light beams are propagated and interfere with one another while maintaining the phase relationship.
If the coherency is high, the laser light interferes with stray light having difference propagation distances, such as scattered light, thus frequently producing the noise due to interference patterns caused by interference of light beams having mutually irregular phase relationship, or the so-called speckles (speckle noise). These speckles are particularly responsible for deterioration in performance in a device for which an illumination system of high uniformity is required, and hence is felt to be obstructive to application of the laser light source to the semiconductor light exposure device.
First, the relationship between the spectrum of laser light and coherency is explained.
Coherency of laser light is represented by a value termed visibility V. This visibility is the contrast of interference fringes formed by dividing laser light into two light beams, delaying one of these light beams so that the proceeding time of the two light beams differs by time .tau. (.tau.=L/C from each other, where L is the light path length difference and C is the velocity of light, and then by synthesizing the light beams. The visibility V is represented by the following equation (1): EQU V(.tau.)=(I.sub.max -I.sub.min)/(I.sub.max +I.sub.min) (1)
where I.sub.max and I.sub.min denote the maximum and minimum values of the strength of the interference fringes, respectively. A light source with V=1 is maximum in coherence and is said to be a fully coherent light source. A light source with V=0 is minimum in coherence and is said to be a fully incoherent light source. In this case, no interference fringes are formed. The distance of coherence or the coherent length L.sub.C denotes the optical path difference of two light beams having sufficiently low visibility V and which can scarcely form interference fringes. The coherent length L.sub.C is defined by the following equation (2): EQU V(.tau..sub.c)=0, L.sub.C .ident..tau..sub.c C (2) EQU V(.tau.)=&lt;(t)E(t)+.tau.)&gt; (3)
On the other hand, visibility (.tau.) coincides with the autocorrelation function of the electrical field E(t) of the laser light, as shown by the equation (3).
From the theorem of Wiener-Khintchine, the autocorrelation function coincides with the Fourier transform of the power spectrum S(f) of the laser light, so that the visibility and the power spectrum are related with each other by the following equation (4): ##EQU1##
It is assumed that the power spectrum S(f) depicts a Lorenz type curve and that the power spectrum is represented by the following equation (5): ##EQU2## where f.sub.0 and .DELTA.f denote the center frequency and the full width of the half value, respectively.
At this time, the visibility V is decreased exponentially as indicated by the following equation (6): EQU V(.tau.).apprxeq.exp(-.pi..DELTA.f.tau.) (6)
The distance for which visibility V becomes sufficiently small is termed coherent length L.sub.C and is defined by the following equation (7): ##EQU3##
FIG. 1A shows, as an example, a Lorenz type curve in which the power spectrum S(f) has the half value full width .DELTA.f=4 G(Hz). FIG. 1B shows the relationship between the optical path length difference L (mm) and visibility V in this case. In FIG. 1B, the abscissa represents the optical path length difference L between the two light beams which is defined by L=.tau.C. The coherent length L.sub.C is on the order of 75 mm.
From the equation (7), coherence and monochromaticity of the laser light are in the relation of trade-off to each other in such a manner that a light source having high monochromaticity, that is narrow spectral width, is high in coherency and susceptible to speckles, whereas a light source having low monochromaticity is low in coherency but large in spectral width, such that the chromatic aberration tends to pose a problem particularly in an ultraviolet light source.
Thus, for applying the laser to, for example, an ultraviolet light exposure device, it is crucial to control the spectral width of laser light to an appropriate value so that no problem will be raised in chromatic aberration nor in speckles.
Various methods have hitherto been proposed for lowering monochromaticity and coherence of laser light.
In a semiconductor light exposure device, attempts have been made towards using KrF excimer laser or an ArF excimer laser, inherently larger in spectral line width and lower in coherency, as its light source.
However, the excimer laser has difficulties that highly toxic gases need to be used, while maintenance costs are high and a larger mounting space is required.
There are also presented difficulties that, because of the excessive spectral line width of laser light, chromatic aberration tends to be produced, such that it becomes necessary to reduce the band of the spectrum of oscillation for preventing the chromatic aberration from occurring. If the spectrum of oscillation is reduced in width, coherency is increased, such that coherency needs to be again lowered.
There is also known a method of employing a solid laser oscillated in multiple modes and to generate ultraviolet light by waveform conversion.
However, in this case, the number of modes or the mode spacing is determined by the structure of laser itself, that is by, for example, physical properties of the laser material. Thus it is difficult to realize the desired coherent length or spectral width in meeting with a particular device to which the light exposure device is applied.
Thus, with the conventional technique, it has been difficult to lower coherency of the semiconductor light exposure device without producing inconveniences.